The invention is concerned with hydraulic dampers for damping vibrations in the fluid flow path between a hydraulic pressure source and an associated hydraulic consumer. The invention is particularly, though not exclusively, applicable for use in a hydraulic actuation system for a clutch on a motor vehicle.
It is known to provide a hydraulic actuation system for operating the clutch of a motor vehicle. Typically such systems comprise a clutch master cylinder which is hydraulically linked with a slave cylinder. The master cylinder is actuated via a pedal located inside the vehicle such that when the pedal is depressed the slave cylinder is actuated to operate the clutch in a known manner. It is also known to provide a hydraulic actuation system for a clutch in which the master cylinder is operated via an electric actuator or in which a hydraulic accumulator is used as a source of hydraulic pressure. Such systems are used to enable automatic or semiautomatic operation of the clutch.
Motor vehicles having a hydraulic clutch actuation system can suffer the disadvantage that vibrations produced in the drive train are transmitted through the hydraulic actuation system. These vibrations may have a detrimental effect on the operation of the actuation system and in manually operated clutches can be felt through the clutch control pedal by a driver of the vehicle. The propagated vibrations can also generate noise in the fire wall of the vehicle and can also lead to pedal growl and clutch roar all which is audible to the occupants of the vehicle.
To attenuate the propagation of these vibrations through the hydraulic actuation system it is known to fit a hydraulic damper in the flow path between the clutch master cylinder, or other hydraulic pressure source, and the clutch slave cylinder to filter out vibrations in the hydraulic fluid. Such a damper is known for example from U.S. Pat. No. 5,320,203. Whilst such dampers have been effective in attenuating the propagated vibrations, there is a need to produce a damper which is both cheap to manufacture and reliable in operation. There is also a need to provide a damper which does not cause an unacceptable increase in the pedal load during disengagement of the clutch or an unacceptable increase in the time taken for the clutch pedal to return to its rest position during clutch re-engagement.
It is an object of the present invention to provide an improved hydraulic damper which meets the above needs.
According to a first aspect of the present invention there is provided a hydraulic damper for damping vibrations in the fluid flow path between a hydraulic pressure source and an associated hydraulic consumer, the damper comprising a cylinder having a bore, a piston slidably received in the bore and having sealing means in contact with the bore to define on one side of the sealing means a first chamber for connection with the hydraulic pressure source and on the other side of the sealing means a second chamber for connection with the associated hydraulic consumer, the cylinder bore having a sealing zone which in a first operative state of the damper is contacted by the sealing means, the piston being capable in response to an increase in hydraulic pressure in the first chamber relative to the pressure in the second chamber of moving in the bore to reduce the volume of the second chamber such that eventually the sealing means moves out of the sealing zone opening fluid flow means through which hydraulic fluid can flow from the first chamber to the second chamber, the piston also being capable in response to an increase in hydraulic pressure in the second chamber relative to the pressure of the first chamber of moving in the bore to reduce the volume of the first chamber such that eventually the sealing means moves out of the sealing zone opening fluid flow means through which hydraulic fluid can flow from the second chamber to the first chamber.
According to a second aspect of the invention, there is provided a hydraulic clutch actuation system having a hydraulic damper according to the first aspect of the invention.